Certain types of marine propellers provide for the discharge of exhaust gases through the propeller, and beneath the water level at a location behind the boat. Propellers of this type include an inner hub, which is connected to the drive shaft of the engine, and an outer hub is spaced radially outward of the inner hub and is connected thereto by a series of radial legs. The space between the hubs defines an exhaust gas passage through which the gas is discharged beneath the water level. A plurality of blades are formed integrally with the outer hub.
To improve the engine and boat performance it has been proposed to attach a diffuser ring to the trailing end of the outer hub. The diffuser ring is flared outwardly and assists in exhaust gas flow and provides a pressure barrier that helps prevent exhaust gases from feeding back into the propeller blades. The outwardly flared diffuser ring thus provides an increase in engine performance.
In the past, the diffuser ring was a separate component, normally stamped from aluminum and attached to the trailing end of the outer hub of the propeller. To provide the attachment, the outer diameter of the outer hub was machined and the stamped diffuser ring was swaged onto the machined surface. This procedure in forming and installing the diffuser ring was costly and time consuming.